Apparatus for repowering and monitoring serial links

ABSTRACT

A computer system employs a repeater unit which repowers a serial channel link. The repeater unit also monitors and records non-idle usage and errors for both directions of the repeated serial link. Non-idle usage of the serial link is recorded as a number of seconds that non-idle traffic flowed in the link over a given period of time. Link serial code violations and loss-of-light transitions are also counted. Link code violations are counted with an accuracy that permits targeted serial link bit-error rates, of no more than one bit error in approximately two months, to be accurately verified for the first time in a normal customer environment. The repeater unit permits an attached monitoring computer to read and reset all its usage and error counters as often as required by the customer, and without losing any counts of any counted event. The attached monitoring computer can also instruct the repeater unit to send certain diagnostic patterns or no-light and/or perform a remote wrap function to assist with link problem determinations.

This application is a division of application Ser. No. 08/376,269 filedJan. 23, 1995 now U.S. Pat. No. 5,572,352, which is a division ofapplication, Ser. No. 08/076,027 now U.S. Pat. No. 5,517,519 filed Jun.14, 1993.

FIELD OF THE INVENTION

This invention relates to data processing apparatus and morespecifically to an apparatus that repowers and monitors traffic on aserial link of the ESCON and FCSI (FCS) type.

GLOSSARY OF TERMS

While dictionary meanings and commonly used meanings are also implied bycertain terms used here, the following glossary of some terms may beuseful.

ESCON

ESCON, as used herein, is based upon the Enterprise Systems CONnectionArchitecture. IBM S/390, but more broadly defines a computer systemsconnection whereby the computer system can execute a channel program fordata transfers and transfer data over a serial interface connecting theserial channel with a serial control unit or another serial channelwhich conforms to a predetermined protocol across a right-of-way pathwaywhich may be distributed through lines that are limited to specifictraffic and, also, through common carrier lines of the kind offered by aRBOC.

RBOC

Regional Bell Operating Company

Serial Link

Connects two serial computer components, and provides a bidirectionalpath between the two components via a serial data conductor in eachdirection.

Multi-mode fiber-optic

The type of fiber-optic conductor driven by an LED transmitter.

Single-mode fiber-optic

The type of fiber-optic conductor driven by an LASER transmitter.

Code Violation (CV)

An invalid bit configuration for a character transmitted on the seriallink.

REFERENCES USED IN THE DISCUSSION OF THE INVENTION

During the detailed description which follows the following works willbe referenced as an aid for the reader. These additional references are:

"IBM Enterprise Systems Architecture/390 ESCON I/O Interface", IBMmanual number SA22-7202. This publication will be cited as "ESCON" or"ESCON Architecture". This Architecture will encompass the similarfunctions performed by like serial link architectures now developed likethe Fibre Channel System Initiative (FCSI) and others which will bedeveloped in the future for linking computer system elements.

U.S. Pat. No. 4,486,739, issued to Franaszek, et al. on Dec. 4, 1984defines the serial characters flowing on the serial link in accordancewith the invention we have developed. We have cited this patent as the"8/10 code" or "8/10 transmission code".

U.S. Pat. No. 4,975,916, issued to Miracle, et al. on Dec. 4, 1990describes synchronization to the serial characters being repoweredthrough the serial link. This patent will be referenced in the text as"character sync".

U.S. Pat. No. 5,107,489, Issued Apr. 21, 1992, to P. J. Brown et al.relating to the "Serial Link Protocol for Forming Dynamic Connections"is an example of one of many patents illustrating other aspects of theESCON and FSCI.

These additional references are incorporated by reference.

BACKGROUND OF THE INVENTION

A general object of this invention is to provide a new and improvedapparatus for repowering and monitoring the traffic on an ESCON seriallink. Serial link repowering apparatus introduced earlier providebackground information that is useful in understanding our invention.

ESCON originates from the IBM named Enterprise Systems CONnection forserial links via fiber-optic channels. It has become a widely adoptedprotocol for network communication. It is used, by IBM and others.Typical network utilize, IBM's SNA traffic however other multipleprotocols can be supported. IBM has developed protocols and prototypesfor T3-capable communications that allow host to host data transfer forMCI Communication Corp. Recently, Hewlett-Packard and Sun Microsystemshave joined IBM to promote a Fibre Channel Systems Initiative. (FSCI) asa multi-vendor standard for linking host devices with each other andwith high capacity storage devices. The proposed Fibre Channel Standardand ESCON are functionally similar. Normally, for IBM compatible systemsESCON is used, while FSCI will be used for channel-speed applicationsinvolving incompatible hosts and peripherals. Several Regional BellOperating Companies (RBOC) have ESCON turnkey services that let usersinterconnect mainframes and peripherals. The FCSI and ESCON allow thecarriers to choose either protocol for functionally similar applicationsand for channel-extension applications. These channels enable connectionto be made for great, and between distributed and clustered systems(SYSPLEX).

ESCON, as used herein, refers to a serial connection system in whichdynamic connections are established in a serial link between computersystem devices. The connections make use of serial links of multi-modefiber-optic or single-mode fiber-optic pathways. ESCON, as used herein,encompasses the same functions achieved by any fiber channel interfacebetween systems, including the FCSI, which permits a computer system toexecute a channel program for data transfers via a serial interfaceconnecting the serial channel with another serial component whichconforms, directly or indirectly, to a predetermined protocol fortransfers across a right of way between system elements. The right ofway is the pathway over which the data transfers via serial links ofmulti-mode fiber-optic or single-mode fiber-optic ribbons. ESCON is usedherein as the common name, since the standard by which this serialconnection system was developed was originated by IBM in its EnterpriseSystem CONnection program. As indicated, this program has been expanded,and now allows disparate elements (heterogeneous systems) to be coupled.The current ESCON Architecture is defined by the referenced "IBMEnterprise Systems Architecture/390ESCON I/O Interface", IBM manualnumber SA22-7202.

As a protocol, connections are made by the use of frames each having abeginning of frame delimiter, an end of frame delimiter, anidentification of the source, and an identification of the destinationof the requested connection. The delimiters may initiate a connect or adisconnect operation between the source and the target. The connectionsare made through a dynamic switch so as to operate the link in one ofsimplex, half duplex or full duplex modes dependent on the number,direction and type of frames. An example of one of the patentsillustrating ESCON is the referenced U.S. Pat. No. 5,107,489 Issued Apr.21, 1992, to Brown, et al. relating to the "Serial Link Protocol forForming Dynamic Connections."

In spite of the great advances being made no one has solved the problemswhich can originate on right of ways, especially those right of waysthat involve common carrier links. Some of those problems, which will bedetailed in the detailed description overview, have been solved by thepresent invention, which relates to an ESCON and FSCI type link.

IBM built a very limited number of prototype fiber-optic serial linkrepowering apparatus during the period 1988 to 1993 with a codename of"PARROT". These PARROT prototypes were installed in a very limitednumber of customer accounts to solve a specific problem of a singleESCON serial link being shorter than the customer's required distance.The PARROT repeater apparatus repowered the serial bit stream, indicatedthat no-light was being received in either direction, and offered aremote wrap capability that transmitted what was received from a linkback into that same link, including the wrapping of no-light received tono-light transmitted. A PARROT has also been built after our inventionto convert from a single-mode (LASER) link to a multi-mode (LED) link.These PARROT apparatus could not indicate that 8/10 code violations(CV's) were being received, that a serial link is being used fortransmitting something other than ESCON 8/10 codes (e.g.--voice), couldnot measure non-idle usage of the link, and could not generate andtransmit any serial bit streams for diagnostic purposes.

A serial fiber-optic repeater is commercially available as the IBMmulti-port 9032 or 9033 ESCON Director. Configuring a pair of pods as adedicated or "static" connection would repower the ESCON serial data andcould also convert from multi-mode to single-mode similar to the PARROT.Diagnostic capabilities exist for a director pod to transmit no-light,idles or any of the four special continuous sequences defined by theESCON Architecture. The director can also configure a pod into remotewrap mode similar to the PARROT, except that a director pod cannotremote wrap no-light received to no-light transmitted. Additionally, thedirector can detect that CV's are being received to the extent that theESCON-architected bit-error-rate (BER) threshold has been exceeded, butthe director cannot count the exact number of CV's that have beendetected. The BER threshold exceeded indication cannot be used todetermine whether the serial link is being used to transmit other thanESCON characters. The director is incapable of measuring non-idle usageof the ESCON link.

SUMMARY OF THE INVENTION

Our invention deals with the use of an apparatus for repowering a seriallink. The references use various names for such apparatus, In thefollowing description, apparatus that is connected between two serialcomponents for the purpose of repowering the serial link will be calledthe repeater unit (or repeater).

The improvements which we have made achieve

The ability to measure the non-idle usage over a period of time for eachdirection of the serial link, and

The ability to count the exact number of CV's detected in each directionof the serial link.

The independent abilities both to transmit various diagnostic patternsin both directions of the serial link and to transmit toward a directionof the serial link whatever is being received from that same direction.

These improvements are accomplished by providing a means to interrogatethe serial characters being repeated in both directions, but withoutdisturbing the repeated serial bit streams. The interrogated repeatedcharacters are tested for being valid characters or code violations(CV's). It the repeated characters are valid, then they are tested forbeing idle or non-idle. A hardware device is used to count the CV's andnon-idle characters. A microprocessor (MP) periodically reads and resetsthe CV counts and adds these counts to its accumulated CV counters. Thehardware device can count exactly one second's worth of non-idlerepeated characters in each direction of the link, and then indicatethis to the MP.

After the MP detects that a second's worth of non-idle usage has beenaccumulated, then the MP will reset this hardware indication and willadd one to its corresponding seconds count.

An attached local monitoring computer (LMC) will periodically poll therepeater to read and reset its CV counts and seconds counts. The LMC addthe counts read from the repeater to its own accumulated CV counts andnon-idle usage seconds counts. The LMC thresholds the total CV countsread over a period of time, and alerts the owner of the repeater andrepeated link that link problem determination should be initiated if aCV threshold is exceeded. It is possible, with our invention, toinitiate this link problem investigation before the problem becomesapparent to the end-point customers that own the serial components atthe ends of the repeated link. Reading and resetting the accumulatedseconds of non-idle usage permits the owner of the repeater and repeatedlink to bill the end point customers a base rate for the link itselfplus an additional rate for the amount of non-idle usage during thebilling period. If the repeated link connects a server to a client, thenthe server could/would bill the client for service based on the numberof non-idle usage seconds accumulated during the billing period.

The LMC can instruct the repeater to transmit certain diagnosticpatterns both to assist in the testing of other attached repeaters andto assist during link problem determination. The LMC can also instructthe repeater to remote wrap whatever is received from a particulardirection of the link back toward that same direction.

These and other improvements are set forth in the following detaileddescription. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview of a repeater configuration example showingmultiple repeaters in the repeated link between the two end-pointcustomers.

FIG. 2 is a block diagram of the preferred embodiment and particularlyshows the repeated serial bit stream with the interrogation of theserial data being repeated.

FIG. 3 shows the code interlocks for usage metering in more detail.

FIG. 4 shows the code interlocks for error monitoring in more detail.

FIG. 5 shows the controls of the diagnostic multiplexer logic block ofFIG. 2 in more detail.

Our detailed description explains the preferred embodiments of ourinvention, together with advantages and features, by way of example withreference to the following drawings.

DETAILED DESCRIPTION OF THE INVENTION

Before considering our preferred embodiments in detail, it may beworthwhile to illustrate, by way of example, some of the typicalproblems encountered in developing systems, before detailing thepreferred embodiment of our invention. The repeater enables ESCONchannel links connecting end-point customers to cross a non-ownedright-of-way, which is usually owned and controlled by a Regional BellOperating Company (RBOC). The RBOC's provide voice, video and datanetworking services across their right-of-way for their (end-point)customers, and the RBOC's require the ability to monitor and control theusage of these serial links passing through their rights-of-way. Ourinvention solves the following problems:

1. Conversion and Distance

Problem: End-point customers usually have multi-mode fiber opticconductors which are driven by LED's and which have a maximum distancerestriction of three kilometers. The distance required to cross an RBOCright-of-way may exceed three kilometers. The fiber optic conductorscrossing the RBOC right-of-way are usually single-mode conductors whichmust be driven by LASER's. A LASER transmitter can generally drive amaximum of 20 kilometers of single-mode fiber optic conductor.

Solution: Our repeater reshapes and redrives the serial bit stream forthe ESCON links, and also provides conversion between multi-mode fiberoptic cables driven by LED's and single-mode fiber optic cables drivenby LASER's. Hence, the repeater may be configured as LED-to-LASER,LASER-to-LASER or LED-to-LED. A plurality of repeaters (e.g. three) maybe inserted in a single ESCON link, allowing the connected end points tobe located up to 80 kilometers apart if all of the repeaters areLASER-to-LASER.

2. Bypass Detection

Problem: An RBOC can lose revenue if their customer leases fiber opticconductors for ESCON digital transmissions, and instead uses them totransmit voice data. This practice is commonly referred to as "bypass".The contract entered between the RBOC and its end-point customers wouldspecify that the leased fiber optic conductors will be strictly used fordigital transmissions associated with an ESCON link.

Solution: Our repeater is designed to repeat a fiber optic bit streamwith a nominal frequency of 200 megabits per second. The repeatermonitors the repeated serial data for valid ESCON encoded 10-bittransmission codes, and will detect a code violation (CV) it a non-ESCONcharacter is detected. The repeater counts the number of CV's detectedin both directions of the link. Each of these two CV counters can countup to 16,777,215 CV's. A CV counter will freeze if its maximum value isreached. Both CV counts are made available to an attached (local)monitoring computer. The RBOC can use the local monitoring computer toread, display and reset the CV counts from any attached repeater.

The targeted bit-error-rate (BER) for an ESCON link is no more than onebit error every 10**15 bits. The repeater can only count CV's, andcannot detect or count individual bit errors. A CV is assumed to haveonly one bit in error, because multiple bit errors within a single10-bit transmission code may not result in a CV. Our repeater isbelieved to be the first device that can count CV's with the accuracyrequired to detect whether or not an ESCON link is meeting its targetedBER.

The ESCON Architecture defines that a bit-error-rate (BER) threshold hasbeen exceeded for a serial dynamic switch pod if at least 12 CV "bursts"occur within a five-minute period, and our repeater is designedaccording to this same specification. A CV burst is defined as one ormore CV's occurring within a 1.5-second sample period. The repeaterdetects whether this architected BER threshold has be n exceeded forboth directions of the links and makes this additional informationavailable to be read and reset by the local monitoring computer. Therepeater contains two CV indicators visible to service personnel, oneindicator for each direction of the link. If either direction of thelink detects at least one CV within a 1.5-second sample period, then therepeater will light the corresponding indicator for a period of 1.5seconds. These CV indicators will be useful to service personnel duringlink problem determination and repair.

Voice data is normally digitized at a much lower bit frequency thanESCON data, which should cause nearly continuous CV's to be detected bya repeater. The number of CV's counted should be orders of magnitudemore than the ESCON architected BER threshold, and the CV indicatorsshould be continuosly. These physical indications and the large countsof CV's read by the local monitoring computer should alert the RBOC tothe possibility that non-ESCON data (e.g.-voice) is being transmittedthrough the link.

3. Monitor Link Errors

Problem: To achieve a high degree of customer satisfaction, an RBOC mustbe cognizant of any errors or outages on the ESCON links crossing itsright-of-way. If a link failure requires corrective action by the RBOC,then this corrective action must be undertaken as soon as possible.Ideally, an RBOC could be already working to correct a link problembefore the problem is noticed by its end-point customers.

Solution: In addition to counting CV errors as described above, ourrepeater also detects for each direction of the link the following othererror information which is useful for link problem determination. Allthis error information can be read by a local monitoring computer forusage by the RBOC:

Loss-Of-Light (LOL) condition: An LOL condition is detected if theoptical signal deteriorates below a predetermined threshold or if allzero bits are being received.

LOL count: The repeater counts up to 255 off-to-on transitions of LOL.An LOL counter will freeze if its maximum value is reached. An attachedlocal monitoring computer can read, display and reset the LOL counts.

Loss-Of-Sync (LOS) condition: LOS is detected because CV's are occurringtoo frequently, and LOS is reset after a significant number oferror-free 10-bit transmission codes are received. An LOS condition isdetected if four or more CV's are detected within 46 or fewer 10-bittransmission codes received. Receiving forty-five consecutive non-CV10-bit transmission codes resets LOS.

Not-Operational Sequence (NOS): The ESCON architecture specifies that anESCON serial component must transmit NOS it a link failure is detected.A link failure is defined as reception of either LOL or LOS for morethan one second. Reception of NOS by a repeater could be caused by thissame repeater being unable to transmit the repeated serial data intothis link. Therefore, the repeater itself could be the cause of the linkproblem if NOS is being received.

Offline Sequence (OLS): The ESCON architecture specifies that an ESCONserial component must transmit OLS while offline, immediately afterpowering on, and immediately prior to powering oft. Correctlytransmitting OLS will prevent a receiving serial component fromdetecting a link failure as a result of receiving LOL from thepowered-off serial component. The RBOC can also instruct an attachedlocal monitoring computer to instruct an attached repeater to transmitOLS into one or both links as a link problem determination debug aid.

4. Measure Bandwidth Usage

Problem: An RBOC may wish to bill its end-point customers for usage ofthe ESCON link similar to telephone billing, with a base charge plus ausage charge. One end-point customer may be providing a server functionfor the other end-point customer and wishes to bill similarly for thebase link hookup plus usage of the link.

Solution: The owner of a repeater can monitor both directions of arepeated ESCON link for non-idle usage. The ESCON architecture specifiesthat idle characters will be transmitted between the serial "frames". Aframe consists of a stand-of-frame delimiter, the frame contentsdata-type characters, two CRC data-type characters, and an end-of-framedelimiter. The frame delimiters consist of multiple "K-characters",which are link control characters and are not data-type characters. Theidle characters between frames are also K-characters and are defined asK28.5 characters. Except for a K28.5 character that begins a certainstand-of-frame delimiter and ends a certain end-of-frame delimiter, allcharacters associated with a frame are considered non-idle characters bythe repeater. The repeater considers all K28.5 characters as idlecharacters. The repeater counts the number of second's worth of non-idlecharacters that flow in both directions in the link. Since each ESCON10-bit serial character is nominally 50 nanoseconds in duration, thenfor every 20 million repeated non-idle characters, the repeater willaccumulate one second's worth of non-idle usage. The repeater canaccumulate up to 16,777,715 seconds (approximately 27.7 weeks) ofnon-idle usage in each direction. A seconds counter will freeze if itsmaximum value is reached. An attached monitoring computer can read,display and reset the repeater's two seconds counters. If link usage isto be billed on a monthly basis, then the billing can be calculated perthe number of non-idle seconds accumulated by the repeater for bothdirections of the link over a period of one month.

The ESCON architecture defines certain special continuous sequences(SCS's) that are used for notifying an ESCON component that an unusualcondition either has occurred or is about to occur. An SCS is defined asa K28.5 character alternating with a particular data-type character(referred to as an "ordered-pair"), and the particular data-typecharacter used defines which SCS is being transmitted. A minimum ofeight consecutive identical ordered-pairs must be received in order tobe deemed a valid SCS. The following SCS's are defined:

Offline Sequence (OLS),

Not-Operational Sequence (NOS),

Unconditional Disconnect Sequence (UD), and

Unconditional Disconnect Response Sequence (UDR).

The UD and UDR sequences are transient in nature, and are used for linkrecovery. The OLS sequence indicates that an ESCON component is offline,and the NOS sequence indicates that a link failure was detected. An OLSor NOS sequence may last indefinitely. When any SCS begins, then therepeater will count the data-type characters that are pad of the firsteight ordered-pairs as non-idle usage, but will stop counting non-idlecharacters if a valid OLS or NOS sequence is received. No more non-idlecharacters will be counted as long as OLS or NOS persists. Since the UDand UDR sequences are transient in nature, then the repeater continuesto count non-idle characters during any UD or UDR sequence.

If either direction of the link is receiving LOL or a CV, then nonon-idle character will be counted for usage metering.

To summarize, each character being received in each direction of therepeated link will be counted for usage metering if:

The character received is NOT a K28.5 character,

LOL is not being received,

A CV is not detected for this character,

A NOS sequence is not being received, AND

An OLS sequence is not being received.

5. Link Diagnostic Capabilities

Problem: Diagnostic capabilities are required both to be able toascertain correct operation of other repeaters add to assist in problemdetermination of the link itself.

Solution: Our repeater has the ability to continuously transmit, underinstructions from a local monitoring computer (LMC), the followingdiagnostic patterns independently in either direction or both directionsof the serial link:

"OLS": Transmit OLS continuously.

"good characters": Transmit continuously an alternating sequence ofvalid 8/10 transmission codes, half of which are idle (K28.5) charactersand half non-idle characters (K28.6).

"LOS": Transmit continuously a repeated 8/10 transmission code (K28.6),which should cause CV's and LOS to be detected by the receiver.Repeating the same 8/10 transmission code having an unequal number ofone bits and zero bits violates the rules of disparity for the 8/10transmission codes. A CV is detected for either an invalid 8/10transmission code or for a disparity error.

"remote wrap": Transmit on a first output line the serial bit streamthat is being received on a first input line and/or transmit on secondoutput line the serial bit stream that is being received on a secondinput line, such that the repeater transmits back to a transmitter theserial bits stream sent to the repeater by that transmitter.

No-light: Transmit no-light, which should result in the receiverdetecting Loss-Of-Light (LOL).

The Preferred Embodiment

Turning now to our invention in greater detail, FIG. 1 illustrates arepeater configuration example showing a single ESCON link 1 passingthrough a maximum of three repeaters, with conversion from multi-mode(MM) to single-mode (SM) fiber optic cables Repeaters #1 & #3 are bothLED/LASER repeaters, and repeater #2 is a LASER/LASER repeater. Eachfiber-optic cable contains two fiber optic conductors/paths (2 & 3), onefor the serial traffic flowing from End-Point Customer X to End-PointCustomer Y (2), and one for the serial traffic flowing from Y to X (3).The repeater technology may limit the maximum number of repeatersbetween two ESCON components, because of serial bit frequency "jitter".Each repeater derives its transmitted bit frequency from the frequencyof the received serial bit stream, and each repeater will introduce aslight amount of bit jitter as a result of this serial repeatermechanism. The serial bit jitter introduced by each repeater becomesadditive when multiple rap eaters are cascaded back-to-back in a link.

The "LMC" (4, 5 or 6) is a Local Monitoring Computer (typically aPersonal Computer or equivalent) that is attached to each group ofrepeaters that are located at the same site. The LMC communicates to itsattached repeaters via an IEEE 488 industry-standard interface 7 that isalso referred to as a General Purpose Instrumentation Bus(GPIB)interface. Using commercially available GPIB extender units, up to196 (14×14) repeaters can be attached to a single LMC GPIB controller.Without GPIB extenders, a maximum of 14 repeaters can be attached to asingle LMC GPIB controller. Using a defined set of GPIB "orders", theLMC can read and reset the accumulated error counts and non-idle usageseconds counts from any attached repeater. A Remote Monitoring Computer(RMC) 8 may be attached via remote links (such as modems and telephonelines) to multiple LMC's. The RMC provides a remote single point ofcontrol and service for all the repeaters owned by a particularright-of-way customer (typically an RBOC). The RMC can analyze errorcounts and status information read from its configured LMC's todetermine the cause of a link problem and dispatch personnel to repairthe problem. At the end of each billing period, the RMC can read thenon-idle usage seconds counts from all the repeaters attached to all itsLMC's and generate the information required for billing purposes. Allthe same error information and usage seconds counts can also be obtainedlocally from an LMC, regardless of whether any remote link existsbetween the LMC and an RMC. This would be the likely mechanism used forbackup if a remote link were down, or if an end-point customer owns oneor more repeaters, needs to obtain error and usage information, and doesnot need to invest in an RMC.

Refer to FIG. 2 for a block diagram of our repeater. Two repeated serialpaths, X-In-to-Y-Out (2a & 2b) and Y-In-to-X-Out (3a & 3b), pass througha repeater. All the hardware necessary to support the path X-In-to-Y-Outis also duplicated for the path Y-In-to-X-Out, and is not shown on thedrawing. A fiber-optic serial bit stream 2a entering the repeater isreceived by a fiber-optic receiver (FORX) 20. The FORX 20 converts theoptical signal to an electrical signal which is passed to thedeserializer (DESER) 21. The DESER 21 deserializes consecutive groups of10 serial bits, and passes these 10 parallel bits, 5 bits at a time, toa "5/5" staging register 22. The DESER 21 also generates a 20 MHzreceiver clock (RCVCLK) derived from the 200 Mbit/sec serial bit stream.This 20 MHz RCVCLK has a nominal 50-nanosecond cycle and a symmetricalup-time and down-time. Each consecutive group of 5 deserialized bits islatched into the "5/5" register 22 by each consecutive plus-to-minus andminus-to-plus transition of the RCVCLK. This same RCVCLK is used to stepthe 10-bit groups from the "5/5" register 22 to the "10" register 23 tothe "6/4" register 24 and into the serializer (SER) 25. The SER 25synthesizes the 20 MHz/sec RCVCLK back into a 200 MHz/sec frequency,which is used to serialize each 10-bit group back into an electricalserial bit stream, one bit every 5 nanoseconds. The connected fiberoptic transmitter (FOTX) 26 converts the electrical serial bit stream toan optical serial bit stream 2b leaving the repeater, which is identicalto the optical serial bit stream 2a that entered the repeater. LOL ispropagated through the repeater by connecting the shown X-In LOL signaldetected by the FORX 20 to the shown "INHIBIT LIGHT" signal entering theFOTX 26. INHIBIT LIGHT causes the FOTX 26 to shut off its output light,which propagates the received LOL condition through the repeater.

Since each consecutive group of 10 serial bits is stable for an entire50-nanosecond RCVCLK cycle in staging register "10" 23, the output ofthis staging register "10" 23 is used for the metering/monitoring logic29 that monitors for error conditions and non-idle usage. The 10parallel bits first enter a character selector (CHARSEL) 27, which iscapable of selecting any one of ten 10-bit "windows" within a stream of19 chronologically consecutive bits. The selected 10-bit characterenters a decoder (DEC) 28, which tests the character is detected forbeing a valid 10-bit transmission code as defined by the ESCONarchitecture. If an invalid character, then the DEC 28 outputs a codeviolation (CV) signal. If the frequency of CV's evolves into an LOScondition, then the DEC 28 will assume that character sync has been lostand will signal the CHARSEL 27 to "bump" to the next 10-bit window, astaught by the aforementioned U.S. Pat. No. 4,975,916. This processcontinues until the DEC 28 finds the correct 10-bit window and stopsdetecting CV's, If a CV is not detected, then the DEC 28 cansuccessfully decode a 10-bit transmission code into a valid data-typecharacter or K-character and output this information via bus 30 to themetering/monitoring logic 29. The metering/monitoring logic 29 receivesLOL from the FORX 20, and receives CV, LOS, bits 0-7, bit K and a paritybit (for bits 0-7 and K combined) via bus 30 from the DEC 28. All thelatches in the CHARSEL 27, DEC 28 and metering/monitoring logic 29 areclocked by the RCVCLK generated by DESER 21.

The metering/monitoring logic 29 counts a second's-worth (20,000,000) ofnon-idle characters, counts CV's, counts off-to-on transitions of LOL,and monitors the current states of LOL, LOS, NOS and OLS. Themetering/monitoring logic 29 decodes the valid special continuoussequences NOS and OLS from the characters being received from DEC 28 viabus 30. It the metering/monitoring logic counts one second's worth ofnon-idle characters, then this is reported to an attached microprocessor(MP) 31 as a "usage-second" indication 34. After the MP 31 detects theusage-second indication, then the MP 31 will reset the usage-secondindication and will increment a large (3-byte) seconds accumulationcounter contained in its local storage (LS) registers (not shown). Anincrement to a seconds accumulation courtier will be ignored if thecounter has already reached its maximum value. The MP 31 can also readand reset the CV count 35 and LOL transition count 36, and can read thecurrent states of LOL, LOS, NOS and OLS from the metering/monitoringlogic 29. The MP 31 adds the CV and LOL transition counts read to largerCV and LOL transition accumulation counters also contained in its LSregisters.

The stored program for the MP 31 is contained in a writeable controlstorage (WCS) of the microprocessor which is loaded from anelectrically-eraseable PROM during a power-on reset initializationsequence. An attached LMC (4, 5 or 6 in FIG. 1) can load new microcodeinto the EEPROM via the GPIB interface 7. The MP 31 also contains asmaller ROM, which contains the code necessary to load the WCS during apower-on reset initialization sequence. An attached LMC can send apre-defined "order" to the MP 31 via the GPIB interface 7. The MP 31will execute the order and send a response back to the LMC via GPIBinterface 7. This order mechanism is used by the LMC for allcommunications with all attached repeaters, such as the reading andresetting of error counts and usage information and the reading ofstatus information.

The MP 31 is able to read, via bus 32, configuration switches presetduring installation to acquire its correct GPIB primary and secondaryaddresses. One additional switch is used to indicate that external wrapdiagnostics through diagnostic wrap plugs should be executed during apower-on reset initialization sequence. Diagnostic wrap plugs areinserted into both the X side and the Y side of the repeater, and wrapthe serial bit streams from Y-Out 2b to Y-In 3a and from X-Out 3b toX-In 2a. The MP 31 can control, via bus 33, indicators on the repeaterto indicate that its diagnostic wrap self-test was successful, that itis not operational, that LOL is being received on X-In 2a or Y-In 3a, orthat a CV burst has been detected on X-In 2a or Y-In 3a.

The diagnostic multiplexer (DIAGMUX) 37 has multiple additional inputsnot shown in FIG. 2, but explained later in reference to FIG. 5. Thesealternate inputs can be selected by the MP 31 for diagnostic purposesduring a power-on reset initialization sequence and under direction ofcertain GPIB orders received via bus 7 from the LMC.

FIG. 3 shows the interlock controls between our repeater hardware and MP31 for the purpose of usage metering. A non-idle character courtier(CHARCTR) 40 is incremented (INC) every 50-nanosecond machine cycle ifLOL is not being received, a CV is not being received, NOS is not beingreceived, OLS is not being received, and a K28.5 (Idle) character is notbeing received, as determined by OR 41 and inverter (N) 42. If theCHARCTR 40 has already reached a value of 19,999,999 (x`1312CFF`) andanother increment occurs, as determined by comparator (CMPR) 44 and AND(A) 45, then a one-second latch (SECLATCH) 43 is set and the CHARCTR 40is reset. The reset (RST DOM) to the CHARCTR 40 dominates over theincrement and forces the CHARCTR 40 to zero. The SECLATCH 43 being on isthe "USAGE SECOND" indication 34 to the MP 31 that exactly one second'sworth of non-idle usage has been accumulated, and the CHARCTR 40 hasbeen prepared to stand accumulating the next second's worth of non-idleusage. This usage metering hardware is duplicated for both pathsX-In-to-Y-Out (2a to 2b) and Y-In-to-X-Out (3a to 3b). The MP 31periodically tests the SECLATCH 43 corresponding to each path, and, ifeither one is active, resets the active SECLATCH 43 via reset 46, andincrements the corresponding 3-byte seconds count contained in its LSregisters. The processing speed of the MP 31 and the structure of the MPcode assure that no usage seconds will be lost. The MP 31 code samplesboth SECLATCH much more frequently than once every second.

FIG. 4 shows the interlock controls between the repeater error-countinghardware and MP 31 for the purpose of counting CV's and LOL off-to-ontransitions. A 24-bit CV counter (CVCTR) 50 is incremented (INC) foreach CV if LOL is not active and the CVCTR is not already full, asdetermined by AND (A) 51 and inverter (N) 52. The CVCTR 50 not full isdetermined by comparator (CMPR) 53 and inverter (N) 54. If the CVCTR 50is incremented every cycle, then it will become full in approximately0.838 second. An 8-bit LOL counter (LOLCTR) 60 is incremented (INC) foreach off-to-on transition of LOL if the LOLCTR is not already full, asdetermined by AND (A) 61. An on-to-off transition of LOL is detected bya one-cycle polarity hold latch inverter (N) 63. When clocked, apolarity hold latch captures the state (on or off) of the input, andholds this same state on its output for one clock cycle. If LOL isactive during the current clock cycle and was inactive during theprevious clock cycle, then this combination will cause the two topinputs to AND 61 to be active for one and only one clock cycle. TheLOLCTR 60 not full is determined by comparator (CMPR) 64 and inverter(N) 65. It is assumed that LOL will come on rarely, and any occurrenceof LOL is considered serious.

The LOLCTR 60 is capable of recording up to 255 LOL off-to-ontransitions. The CV and LOL-transition counting hardware is duplicatedfor paths X-In-to-Y-Out (2a to 2b) and Y-In-to-X-Out (3a to 3b).

The MP 31 periodically reads and resets all the hardware error counters,and adds the error counts read to the corresponding error countscontained in its LS registers. The MP 31 reads the hardware errorcounters by generating a read/reset signal 55, which is transformed byhardware a pulse with a duration of exactly one RCVCLK clock cyclewithin each hardware domain that contains error counters. The MP 31 isclocked by a system clock (SYSCLK), which is also nominally 20 MHz(50-ns clock cycle). The SYSCLK is asynchronous to both the RCVCLK'sthat are generated within the domains of each repeated path, and theRCVCLK's are also asynchronous with respect to each other. The MPgenerates this single read/reset signal 55 to read all the CV andLOL-transition counts from both the X-In-to-Y-Out (2a to 2b) andY-In-to-X-Out (3a to 3b) paths. The following explanation only describesthe reading of a CV count, but the mechanism used to read all CV countsand LOL-transition counts is identical to this description. Theread/reset pulse 55 both gates the contents of the CVCTR 50 into itscorresponding CV backup (CVBKUP) register 56 and resets the CVCTR 50.The reset (RST DOM) to the CVCTR 50 dominates over any increment thatmay also be active during this same clock cycle, and forces the CVCTR 50to zero. However, if an increment is active during the same cycle as theread/reset pulse, then this fact is detected by AND (A) 57 andremembered for one additional cycle in the one-cycle polarity hold (PH)latch 58. The output of a one-cycle PH latch for any given cycle equalsthe input to th PH latch during the previous cycle. If the output of thePH latch 58 is active, then this increments (once only) the contents ofthe CVBKUP register 56 to compensate for the increment that coincidedwith the read/reset pulse and that would have otherwise been lost. TheMP delays a sufficient duration of time to allow the CVBKUP register 56to increment, if required, and then reads via bus 35 the stable contentsof the CVBKUP register 56: The MP 31 adds the value read from the CVBKUPregister 56 to the corresponding CV count contained in its LS registers.

FIG. 5 shows the interlock controls between the repeater diagnostichardware and the MP 31 for the purpose of either allowing normaloperation, forcing a particular pattern to be transmitted, or activatinga remote wrap. All the hardware devices shown for the X-IN-to-Y-OUT (2ato 2b) path are duplicated for the Y-IN-to-X-OUT (3a-to-3b) path and arecontrolled independently by the MP 31. Using a writeable hardwareregister and decoder (not shown), the MP 31 is capable of activating oneand only one signal in signal group 75 (sel A, sel B, sel C, sel D, selE or sel F). Activating "sel A" 75 allows for normal operation by gatingthe "NORMAL" input A through the DIAGMUX 37. The RCVCLK 79 generated byDESER 21 drives the "5/5" register 22 and "10" register 23, and willalso drive the "6/4" register 24 and SER 25 via CLKMUX 73 in normalmode. In normal mode, the signal "sel A" 75 activates "sel a" in signalgroup 76, which gates RCVCLK 79 input "a" through CLKMUX 73 to the "6/4"register 24 and SER 25.

Activating "sel B" 75 or "sel C" 75 or "sel D" 75 activates "sel b" 76via OR (O) 72, which gates SYSCLK input "b" through CLKMUX 73. The morestable SYSCLK is used for the generation of all three diagnosticpatterns ("OLS", "good characters" and "LOS"), therefore SYSCLK must begated through the CLKMUX 73 when these three diagnostic patterns arebeing generated.

Activating "sel B" 75 forces a diagnostic pattern of OLS to betransmitted by gating input B through DIAGMUX 37. MUX 70 generates theOLS pattern by alternating the -D24.2 (top) input and the +K28.5(bottom) input every other SYSCLK cycle. The "SYSCLK/2" gate to MUX 70and to MUX 71 is active every other (alternate) SYSCLK clock cycle. Therepeated alternating sequence of D24.2 and K28.5 is defined as the OLSsequence by the ESCON Architecture, and indicates an offline conditionto an attached ESCON serial component. Receiving OLS prior to detectionof a link failure condition will prevent an ESCON serial component fromreporting a link incident as a result of tile link failure. The ESCONArchitecture defines that a link failure must be detected if an LOS orLOL condition persists for longer than one second. The capability ofsending OLS via the repeater's DIAGMUX 37 is useful during link problemdetermination, and is also useful to test the OLS-detection capabilitiesof other attached repeaters and ESCON serial components.

Activating "sel C" 75 forces a diagnostic pattern of "good characters"to be transmitted by gating input "C" through DIAGMUX 37. MUX 71generates the "good characters" pattern by alternating the +K28.5 (top)input and the -K28.6 (bottom) input every other SYSCLK cycle similar tothe operation of MUX 70. This "good characters" pattern can be used totest the usage metering capabilities of other repeaters, because thispattern is not OLS, is not NOS, and contains non-idle characters(K28.6's).

Activating "sel D" 75 forces an "LOS" diagnostic pattern of repeated-K28.6's to be transmitted by gating input "D" through DIAGMUX 37. Sincethe -K28.6 10-bit transmission code contains an unequal number of ones(4) and zeros (6), then this should cause the receiver of this patternto detect continuous CV's because the 8/10-code running disparity willconstantly be driven beyond the acceptable limits. Detecting continuousCV's should rapidly cause LOS to be detected, hence this diagnosticpattern is referred to as the "LOS" pattern. This "LOS" pattern an beused to test the LOS-detection and CV-counting capabilities of otherrepeaters.

Activating "sel E" 75 activates the remote wrap function by gating the"remote wrap" input "E" through DIAGMUX 37. This "remote wrap" input "E"comes from the output of the alternate "10" register, which is the "10"register in the alternate path Y-IN-to-X-OUT (3a-to-3b) corresponding to"10" register 23. This transmits on Y-OUT 2b whatever is being receivedon Y-IN 3a, hence the "remote wrap" connotation. Activating "sel E" 75also activates the "sel c" 76 gate to CLKMUX 73, which gates thealternate RCVCLK input "c" through CLKMUX 73 to the "6/4" register 24and SER 25. The alternate RCVCLK is the RCVCLK generated by the DESER inpath Y-IN-to-X-OUT (3a-to-3b) which corresponds to RCVCLK 79 generatedby DESER 21. This alternate RCVCLK must be used to synchronize theentire remote wrap path and maintain integrity of the repeated serialbit stream. Placing our repeater in remote wrap mode can facilitateproblem determination of the attached serial component and link that areremotely wrapped through the repeater.

The output of LOLMUX 74 controls the INHIBIT LIGHT signal to FOTX 26.Activating INHIBIT LIGHT causes FOTX 26 to stop transmitting light,which should cause the attached serial component to detect LOL.Activating "sel A" 75 activates the "sel a" 77 gate to LOLMUX 74, whichgates the LOL 78 input "a" through LOLMUX 74. Gating LOL 78 to FOTX 26assures that no-light received results in no-light transmitted for thenormal path. Activating either "sel B" 75, "sel C" 75 or "sel D" 75activates, via OR 72, the "sel b" 77 gate to LOLMUX 74, which gates the"0" input "b" through LOLMUX 74. This "0" input is an inactive level,and assures that INHIBIT LIGHT is held inactive when transmitting thediagnostic patterns "OLS", "good characters" and "LOS". Activating "selE" 75 activates the "sel c" 77 gate to LOLMUX 74, which gates thealternate LOL input "c" through LOLMUX 74 to the FOTX 26. The alternateLOL is the LOL generated by the FORX in path Y-IN-to-X-OUT (3a-to-3b)which corresponds to FORX 20. This alternate LOL must be used topropagate no-light received to no-light transmitted through the remotewrap path.

Activating "sel F" 75 forces the diagnostic transmission of no-light.Activating "sel F" 75 activates the "sel d" 77 gate to LOLMUX 74, whichgates the "1" input "d" through LOLMUX 74. This "1" input is an activelevel, and assures that INHIBIT LIGHT is held active during thediagnostic transmission of no-light. Note that the data being passedthrough DIAGMUX 37 and clock being passed through CLKMUX 73 are bothunimportant during the diagnostic transmission of no-light, because noserial bit stream is being transmitted.

Computer System Implementation

With our repeater units, elements of a distributed computer system whichhas a client and server can be configured over larger distances. Theconfigured system will solve the problems noted above. Such a systemwill have a serial channel for executing a channel program for datatransfers ("ESCON Channel" in FIG. 1) and a serial interface connectingsaid serial channel with another serial component which conforms to apredetermined protocol (1 in FIG. 1). One or more repeater units willconnect into the serial link between the serial channel and other serialcomponent (any "REPEATER" in FIG. 1 ), and will repower the serial bitstreams passing in both directions on the serial link (20, 21, 22, 23,37, 24, 25 & 26 in FIG. 2). The repeater unit, in accordance with ourpreferred embodiment will have a monitor for measuring and reportingnon-idle usage of the serial link in both directions (FIGS. 2 & 3).

The repeater unit will have a way of interrogating the serial charactersflowing in the s rial link (20, 21, 22, 23, 27 & 28 in FIG. 2) and ofdecoding said serial characters as valid (28 in FIG. 2) and either idleor non-idle characters according to the protocol of the serial link (29in FIG. 2), and provide a counter for counting up to one second's worthof non-idle characters according to the nominal serialization time ifeach serial character (41, 42, 40, 44 & 45 in FIG. 3). The system willdetect when exactly one second's worth of non-idle characters have beencounted (44 & 45 in FIG. 3), will indicate to a higher-level recordingmeans (31 in FIG. 2) within the repeater the fact that one second'sworth of non-idle characters have been counted (43 in FIG. 3 and 34 inFIGS. 3 & 2), and will reset the counter in such a manner that nonon-idle characters can be repeated without being counted (42, 40, 44 &45 in FIG. 3). The recorder, preferably in the form of a microprocessor31 in FIG. 2, will rapidly detect an indication that one second's worthof non-idle characters have been counted (34 in FIGS. 2 & 3). Themicroprocessor will then reset the indicator (46 in FIG. 3) and willincrement a seconds counter capable of holding several months worth ofseconds. The microprocessor will respond to a request to read and resetthe seconds counts from a monitoring device external to the repeater viaa bus (7 in FIGS. 2 & 1 ), passing the seconds counts to the externalmonitoring device, and resetting the seconds counts after passing themto the external monitoring device.

Any repeater unit will also provide error monitoring by counting andreporting both all serial characters having bit errors and allloss-of-light transitions detected in both directions of the serial link(FIGS. 2 & 4).

In this embodiment, the repeater unit will interrogate the serialcharacters flowing in the serial link (20, 21, 22, 23, 27 & 28 in FIG.2), and provide for decoding said serial characters and detectingwhether a decoded serial character contains one or more bits in errorknown as a code violation (28 & 30 in FIG. 2). Upon detection of suchcode violations the repeater unit will provide for incrementing a codeviolation counter capable of holding a very large count of codeviolations if loss-of-light is inactive and said counter is not full(51, 52, 50, 53 & 54 in FIG. 4). The repeater provides viamicroprocessor, a higher-level recording means (31 in FIG. 2) to rapidlyread and reset the code violation counters (55 in FIG. 4) in such amanner that each code violation detected will be counted once and onlyonce and a code violation cannot occur without being counted (50, 57,58, 56 & 35 in FIG. 4). The microprocessor which provides a higher-levelrecording means is capable of

1. adding the code violation counts read to its larger code violationaccumulation counters,

2. responding to a request to read and reset the code violationaccumulation counts from a monitoring device external to the repeatervia a bus (7 in FIGS. 2 & 1),

3. passing the code violation accumulation counts to said externalmonitoring device, and

4. resetting the code violation accumulation counts after passing themto the external monitoring device.

Preferably, the repeater unit will detect loss-of-light from eitherdirection in the serial link (20 in FIG. 2) and will have a way ofdetecting that loss-of-light has transitioned from the off state to theon state (62 & 63 in FIG. 4) and for incrementing a loss-of-lighttransition counter capable of holding a large count of loss-of-lighttransitions if an off-to-on loss-of-light transition is detected andsaid counter is not full (61, 60, 64 & 65 in FIG. 4). The microprocessorwill provide a higher-level recording means (31 in FIG. 2) to rapidlyread and reset the loss-of-light transition counter (55 in FIG. 4) insuch a manner that each off-to-on loss-of-light transition detected willbe counted once and only once and an off-to-on loss-of-light transitioncannot occur without being counted (60, 67, 68, 66 & 36 in FIG. 4) saidhigher-level recording means is capable of

1. adding the loss-of-light transition counts read to its toss-of-lighttransition accumulation counters,

2. responding to a request to read and reset the loss-of-lighttransition accumulation counts from a monitoring device external to therepeater via a bus (7 in FIGS. 2 & 1),

3. passing the loss-of-light transition accumulation counts to saidexternal monitoring device, and

4. resetting the loss-of-light transition accumulation counts afterpassing them to the external monitoring device.

Any repeater unit will also provide diagnostics by independentlygenerating and transmitting multiple diagnostic serial bit patterns orno-light in both directions of the serial link (FIG. 5). The repeaterwill provide means to diagnostic wrap either the serial bit stream orno-light condition received from an attached serial component as thesame serial bit stream or no-light condition, as appropriate,transmitted back to the same attached serial component.

For diagnostics, the microprocessor, in the form of a higher-levelrecording means (31 in FIG. 2) activates various diagnostictransmissions independently into either or both directions of therepeated serial link (FIG. 5) including:

1. An architected special continuous sequence indicating an offlinecondition (75(sel B), 37(B), 70, 24, 25 & 26 in FIG. 5),

2. A repeated pattern of alternating idle and non-idle characters(75(sel C), 37(C), 71, 24, 25 & 26 in FIG. 5),

3. A repeated pattern capable of causing both code violations and aloss-of-character-synchronization condition in the receiver (75(sel D),37(D), 24, 25 & 26 in FIG. 5) or

4. No-light (75 (sel F), 77 (sel d), 74(d) & 26 in FIG. 5),

The repeater unit substitutes stable system clocks for usage during allof the aforementioned diagnostic transmissions except the diagnostictransmission of no-light, which requires no clocking (75(sel B or C orD), 72, 76(sel b) & 73(b) in FIG. 5), and inhibits the transmission ofno-light regardless of no-light being received during all of theaforementioned diagnostic transmissions except the diagnostictransmission of no-light (75(sel B or C or D), 72, 77(sel b) & 74(b) inFIG. 5).

For a remote wrap, the repeater unit microprocessor (31 of FIG. 2)activates an alternate diagnostic remote wrap mode independently oneither side or both sides of the repeater. A serial b it pattern orno-light condition received from an attached serial component istransmitted unchanged as the same serial bit pattern or no-lightcondition back to the same attached serial component that eitheroriginated or repowered the serial bit pattern or no-light condition(75(sel E), 37(E), 24, 25 & 26 in FIG. 5). The remote wrap moderetransmits the wrapped serial bit stream at the same serial bitfrequency as the serial bit frequency of the received serial bit stream(75(sel E), 76(sel c), 73(c), 24 & 25 in FIG. 5), and transmits no-lightif no-light is being received from the remote wrap path instead of fromthe normal repeated path (75(sel E), 77(sel C), 74(c) & 26 in FIG. 5).

Our invention will be used for a 9036 Remote Channel Extender, whichallows customers to use fiber optic links provided by a carrier, tosites up to 80 kilometers away. A TCP/IP gateway will allow thisprotocol to be used to interconnect devices via ESCON and FCSI. As aresult, ESCON (including its functionally equivalent FCSI portion) canpermit a mainframe to act as if it were a client-server and performclient-server applications with remote devices. The IBM mainframe, as aresult, has the capacity to act an a hierarchical mainframe in thetraditional sense, but also as a server, both for other servers andclients, in a distributed environment that can have elements many milesaway. These processors can work in parallel with general purposemainframes. The 9036 Remote Channel Extender allows connecting processorat distances of up to 80 kilometers using ESCON and the fiber lines ofcommon (e.g. RBOC) carriers.

While we have described our preferred embodiments of our invention, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first disclosed.

What is claimed is:
 1. An apparatus for use with a serial channelinterface connecting a serial channel with a serial component saidserial channel interface conforming to a predetermined protocol, saidapparatus comprising:a repeater unit having a connector for connectinginto a serial link between the serial channel and the serial component,said repeater unit repowering a plurality of serial bit streams passingin both directions on the serial link, wherein the apparatusinterrogates serial characters being repeated in both in bothdirections, but without disturbing the repowered serial bit streams,wherein the interrogated repeated serial characters are tested for beingvalid characters or code violations (CV's), and if the repeated serialcharacters are valid, then they are tested for being idle or non-idle,and wherein the repeater unit reshapes and redrives the serial bitstream for serial links, and also provides conversion between multi-modefiber optic cables driven by LED's and single-mode fiber optic cablesdriven by LASER's whereby the repeater may be configured asLED-to-LASER, LASER-to-LASER or LED-to-LED, and wherein a plurality ofrepeater units may be inserted in a single serial link, allowingconnected end points to be extended apart.